This invention relates generally to polyurethane formulations. More particularly, it relates to a water-compatible urethane-containing amine hardener which produces a polyurethane when combined with an epoxy resin.
Polyurethanes are high molecular weight compounds which have a high degree of strength, hardness, and friction resistance. They are often used as adhesives, cements, and coatings. They are made of polymers which contain repeating urethane groups, as shown in FIG. 1.
Traditionally, polyurethanes may be produced by reacting diols with di-isocyanates, as shown in FIG. 3. U.S. Pat. No. 4,401,499 by Kaneko et. al discloses a method for producing a resin which reacts molecules containing hydroxyl groups with di-isocyanates to form temporary urethane polymers, which are noted for their stability and strength. Isocyanates, however, are highly toxic, non-stable substances because they react easily with water, such as moisture in the air. This method of producing polyurethane cannot be used in many applications, namely those involving direct contact with water.
U.S. Pat. No. 5,175,231 by Rappoport, incorporated herein by reference, describes a method for producing water-compatible polyurethanes which involves reacting oligomeric cyclocarbonates with diamines. This method begins with aliphatic polyepoxy molecules, which are used as precursors for cyclocarbonate-containing molecules. An example is Heloxy 84(copyright) produced by Shell Chemical Company. This product contains several epoxy functional groups. Since a desirable polyurethane coating comprises molecules attached to each other in a non-dissipated, three-dimensional network, it is preferable to have a precursor containing as many multi-functional epoxy groups as possible. However, the aliphatic polyepoxy molecules normally contain a number of residual hydroxyl groups that were not converted to epoxy groups, as epoxidation of aliphatic molecules is never 100% efficient due to the nature of the reaction commonly used. As a result, there are fewer epoxy groups available than desired, thus reducing the number of cyclocarbonate groups and possible links in the future polyurethane network. Although it is possible to increase the percentage of epoxy groups, it is more expensive and technically difficult to reach comprehensive epoxidation of aliphatic hydroxyl-containing compounds such as Heloxy 84(copyright).
In Rappoport""s method, the epoxy functional groups of Heloxy 84(copyright) are reacted with carbon dioxide to produce cyclocarbonate functional groups. The reaction is shown in FIG. 2. This reaction is also less than 100% efficient, leaving some epoxy groups unreacted. Like the residual hydroxyl groups mentioned above, these unreacted epoxy groups reduce the functionality of the urethane molecule and as a consequence, the number of links in the final polyurethane network. Typically, the conversion rate of epoxy groups to cyclocarbonate groups is only about 80-85% efficient at the soft conditions before xe2x80x9cstickingxe2x80x9d of the carbonation reaction occurs. In order to achieve comprehensive carbonation, a more extreme version of the reaction must be carried out. The temperature is raised from 100xc2x0 C. to 130xc2x0 C., the reaction time is increased from 1-1.5 hours to 5-6 hours, and a larger amount of catalyst, usually quaternary ammonium salts, is used. Though this reaction ensures that nearly all the epoxy groups have been turned into cyclocarbonate groups, it also produces undesirable side reactions and products. In addition, it is more expensive and time-consuming.
After the formation of cyclocarbonate functional groups, the molecules are reacted with diamines, such as Vestamine IPD (isophorone diamine) and Vestamine TMD (trimethyl hexamethylene diamine), both made by Hxc3xcls America, Inc. The reaction is shown in FIG. 4. These diamines contain two amine groups with different reactivities. For the isophorone diamine, the aliphatic amine groups are the more reactive amines, while the cycloaliphatic amine groups are the less reactive amines. The more reactive aliphatic amines are usually used to react with the cyclocarbonate groups of the molecules, thus forming urethane links. The less reactive cycloaliphatic amines are left unreacted. The urethane links form the basis for the urethane-containing amine hardener. The amine hardener is usually packaged and stored until it is time to create the polyurethane.
In order to create the polyurethane, the urethane-containing molecules of the amine hardener containing the unreacted less reactive cycloaliphatic amines are combined with an epoxy resin. These less reactive cycloaliphatic amines react with the epoxy resin to form the polyurethane. The polyurethane is then cured as a result of the hardener""s multifunctionality. Unfortunately, because all the more reactive amine groups have previously reacted, there is often a shortage of less reactive amine groups in the curing stage which leaves the reaction incomplete and weakens the structure of the final polyurethane.
In many epoxide resin-amine hardener formulations, reactions are carried out in the presence of organic solvents, which are volatile air pollutants and sometimes carcinogenic. These organic solvents also decrease the reactivity of the functional groups, thus reducing the degree of cross-linking.
Accordingly, it is the primary object of the present invention to improve the efficiency and lower the cost of amine hardener formulations and to overcome problems due to the incomplete epoxidation and carbonation reactions. It is another object of this invention to provide a variety of amine hardeners by formulating different combinations of the necessary structural units, which also allows control over the properties of the polyurethane to be produced. It is another object of the invention to remove hazardous components from the presence of polyurethane users at the final processing stage. Another object of the invention is to produce a superior polyurethane formulation, which is water-compatible, non-toxic, has a low viscosity, and has a high degree of penetrance into a surface (mainly porous) before curing, and is impact-resistant, abrasion-resistant, chemical-resistant, strong, and flexible after curing. It is a final object of the invention to provide a one-package polyurethane formulation, whereby the urethane-containing amine hardener and epoxy resin can be packaged together for a certain amount of time without reacting until needed.
These objects and advantages are attained by an improved urethane-containing amine hardener synthesis. Precursor aliphatic polyepoxies, such as Heloxy 84(copyright), contain a plurality of epoxy functional groups, as well as residual hydroxyl functional groups that were not converted to epoxy groups at the time of the Heloxy 84(copyright) synthesis from polyepoxy molecules. The proposed formulation for amine hardener synthesis makes use of the unconverted hydroxyl groups by reacting them with isocyanate groups on a prepolymer molecule to form a urethane-containing molecule. Although this reaction contains hazardous components, it is achieved under the supervision of specialists in sealed chemical equipment. Polyurethane users are not exposed to any chemical hazards.
As a result of the above modification, the epoxy-containing molecules bearing the mentioned hydroxyl groups are combined together by use of the prepolymer molecule. Consequently, the common functionality of the mixture is increased and a more complete, non-dissipated, three-dimensional network can be created at the curing stage. As is described in the known method, the epoxy groups are reacted with carbon dioxide to form cyclocarbonate groups. If this reaction is carried out at the more soft conditions, it is 80-85% efficient, thus leaving some epoxy groups unconverted. The proposed formulation for amine hardeners is able to make use of the unreacted epoxy groups by taking advantage of the different reactivities of diamine molecules, cyclocarbonate molecules, and epoxy molecules. Aliphatic amines have a high reactivity to both cyclocarbonate and epoxy functional groups, as shown in FIGS. 5 and 6. Cycloaliphatic amines have a lower reactivity but are still able to react with both cyclocarbonate and epoxy functional groups, as shown in FIGS. 7 and 8. Aromatic amines are the least reactive, as they are only able to react with epoxy functional groups, as shown in FIG. 9, but not with the cyclocarbonate groups. Thus, selectively reacting the aromatic amine groups with the unconverted epoxy groups on the urethane molecule renders them functional, but does not affect the cyclocarbonate groups. This reaction produces functional amine-containing molecules which are indifferent to cyclocarbonate groups at the ambient temperature, so that the two can coexist. However, after the addition of the epoxy resin to form the final polyurethane, the aromatic amines will be able to participate in the curing process.
The different reactivity of the amines is also used in the final stage of urethane-containing amine hardener synthesis. Modified diamines are used instead of the xe2x80x9cvirginxe2x80x9d ones used in the known method. By blocking the more reactive aliphatic amine groups of the isophorone diamine with a ketone, thus forming an amino-ketoxime, as shown in FIG. 10, it is possible to allow the less reactive cycloaliphatic amine groups to react with the cyclocarbonate functional groups first.
Interior urethane links are formed in this way. These molecules are stable and can be kept for a certain amount of time until the polyurethane is produced, due to the xe2x80x9chiddenxe2x80x9d more reactive aliphatic amine groups. These molecules also contain xe2x80x9chiddenxe2x80x9d hydroxyl groups near the urethane links which impart water-compatibility to the final hardener as a result of the urethane reaction.
To produce the final amine hardener which can react with an epoxy resin, the more reactive aliphatic amine groups must be deprotected. This is easily achieved with hydrolysis by water to remove the ketones. The regenerated more reactive aliphatic amine groups can then react with the epoxy functional groups of the epoxide resins. Any commercial aromatic epoxy resin may be used. In addition, the resulting amine hardener may be used in combination with other commercial polyamine hardeners such as di-ethylene-triamine or amino-amides to form amine hardeners with different characteristics.
The final amine hardener can be combined with an epoxy resin to form an especially water-compatible formulation which possesses impact-resistance, abrasion-resistance, chemical-resistance, strength, and flexibility after curing. This non-toxic polyurethane has a low viscosity and a high degree of penetrance into a surface, and as such can be used to coat, protect, and repair concrete, cement, wood, gypsum, and other porous surfaces. It can also be used for impregnation and reinforcement. Other uses include water-diluted coatings for wood and water-borne adhesives for silicate materials. Curing can take place at the ambient temperature. Polyurethane coatings produced by this invention are especially strong and flexible, due to the incorporation of the prepolymer molecules which form additional links in the final network, as well as increasing the functionality of the amine hardener.
In addition, the ability of this invention to make use of all three cyclocarbonate, epoxy, and hydroxyl functional groups not only increases the number of links in the final complete network, but also reduces time and cost factors in the synthesis of urethane-containing amine hardeners and the processing of amine hardener-epoxy resin formulations.